For a multitude of applications it is necessary to provide total or substantial sealing among fluids. Sealing requirements become more complex where motion among components must be anticipated, such as that due to differential thermal expansion and seismic excitation, particularly where distortion of the sealing surfaces or uneven motion in more than one direction or plane is probable.
A particular example of complex sealing criteria is that presented by the interaction of the thermal liner and the outlet nozzle of a liquid-cooled nuclear reactor vessel. Such systems provide a bypass stream of reactor coolant which flows upwardly adjacent the primary reactor vessel and then about a portion of the outlet nozzle bounded by a nozzle liner. The primary purpose of the bypass flow is to protect the vessel and nozzle walls from severe thermal transients and excessively high temperatures. Accordingly, the cooler bypass flow must be substantially segregated from the hotter primary coolant which is discharged from the vessel through the nozzle and nozzle liner. The bypass flow passes about the circumference of the nozzle liner, and the primary flow passes through the circular flow area of the nozzle and its liner. The thermal liner is spaced from the vessel and outlet nozzle to define the bypass flow region.
This arrangement has been found to result in significant amounts of non-uniform motion among the thermal liner, vessel and nozzle liner, on the order of 3/4 of an inch in several planes, as well as distortion of potential sealing surfaces into non-planar configurations. Sealing among the flows must not only accommodate the non-uniform relative motion, but also must operate satisfactorily, without maintenance, for the full life of the reactor, approximately 30 years. The seal must maintain resistance to leakage while exposed to high temperature liquid sodium, and must not be overstressed or impose excessive forces on the components with which it interacts.
For a number of years a plurality of alternative designs have been proposed for the sealing arrangement. The concepts considered have included welded joints, omega seals, metallic bellows, piston rings, labyrinth seals, and spring loaded shoes. However, each of these concepts has presented some undesirable characteristics for the intended service. For example, welded joints do not permit the required relative motion of the components without excessive stresses and a likelihood of rupture. Omega seals and metallic bellows cannot sustain the required directions and magnitudes of motion within allowable stress limits. Piston rings and spring loaded shoes are subject to jamming and, over the required maintenance-free lifetime, eventual excessive leakage, as well as possible spring failure. And, labyrinth seals tend to control leakage, rather than prevent it, particularly where leakage flow is unevenly distributed due to non-uniform relative motions.
Thus, it is desirable to provide sealing arrangements which will effectively seal among fluids where the components defining the flow paths of the fluids are subject to substantial multi-directional relative motion.